A door that is much more satisfied with its functionality than it has any right to be.

This kind of device is classic Adams; why restrict the characters to the people or sentient beings? You can instill personality in any device, if you really want to.

“Listen,” said Ford, who was still engrossed in the sales brochure, “they make a big thing of the ship's cybernetics. A new generation of Sirius Cybernetics Corporation robots and computers, with the new GPP feature.”

“GPP feature?” said Arthur. “What's that?”

“Oh, it says Genuine People Personalities.”

“Oh,” said Arthur, “sounds ghastly.”

A voice behind them said, “It is.” The voice was low and hopeless and accompanied by a slight clanking sound. They span round and saw an abject steel man standing hunched in the doorway.

“What?” they said.

“Ghastly,” continued Marvin, “it all is. Absolutely ghastly. Just don't even talk about it. Look at this door,” he said, stepping through it. The irony circuits cut into his voice modulator as he mimicked the style of the sales brochure. “All the doors in this spaceship have a cheerful and sunny disposition. It is their pleasure to open for you, and their satisfaction to close again with the knowledge of a job well done.”

As the door closed behind them it became apparent that it did indeed have a satisfied sigh-like quality to it. “Hummmmmmmyummmmmmm ah!” it said.

The door's "straight man," so to speak, is a very doleful robot, one which was built before emotional balance in robots was really perfected. In the above quotation, the robot Marvin is reading with an overload of irony.